Chlorine Stable Isotope Research Articles

Chlorine isotopes as tracers of solute origin and age of groundwaters from the Eastern Desert of Egypt

Chlorine-36 abundances and stable chlorine isotope ratios (37Cl/35Cl) were analyzed to decipher sources of chloride and to estimate subsurface residence times of groundwaters from the Nubian Sandstone Aquifer System (NSAS), alluvial aquifers, and fractured basement aquifers in the Eastern Desert of Egypt. Chlorine-36 isotopic abundances (36Cl/Cl) ranged from 5 × 10−15 to 343 × 10−15. The range in δ37Cl values was −0.61 to +0.90‰. Groundwater solute compositions were dominated by Na, Ca, Cl, and SO4 with total dissolved solids ranging from 445 to 13,700 mg/L. Chemical and isotopic data indicate that dry fallout of marine aerosols and evaporated vadose-zone residues from precipitation that recharged the aquifers may have been the main sources of groundwater solutes. Apparent groundwater 36Cl model ages range from 0 (+/−40) to 1300 (+300/−180) kyr. Groundwater from the fractured basement aquifers in the southwestern part of the study area had the youngest apparent ages, while that from deep wells tapping the Nubian aquifer had the oldest apparent ages. Shallow alluvial aquifers having anomalously old apparent 36Cl model ages indicate mixing of old Nubian aquifer water with modern recharge in alluvial aquifers. Stable isotope ratios of hydrogen and oxygen support this inferred mixing relationship and allowed quantitative estimation of old/young water mixing fractions.

The origin of the warm groundwater near Lake Mývatn, NE Iceland, traced by stable isotopes

The origin of the warm groundwater which feeds Lake Mývatn is unknown, but it has been affected by volcanic episodes as well as geothermal activity and utilisation. In this contribution stable isotopes of hydrogen (2H), oxygen (18O), sulphur (34S), chlorine (37Cl) and strontium (86Sr and 87Sr) in 20 groundwater and effluent samples from the Lake Mývatn area are used to constrain the origin of the warm groundwater. The results suggest that the warm groundwater is partly formed by mixing with geothermal effluent water and partly by mixing with geothermal steam.

Chemical, microstructural and chronological record of phosphates in the Ksar Ghilane 002 enriched shergottite

The enriched basaltic (martian) shergottite Ksar Ghilane (KG) 002, discovered in 2010, is exceptionally rich in coexisting but discrete apatite and merrillite crystals. It has been selected to better constrain the formation conditions and post-crystallization processes, and thus the evolution of martian rocks based on Ca-phosphates. A petrological, chemical, chronological and microstructural approach using a series of high-spatial resolution techniques including Raman spectroscopy, electron microscopy (SEM, EPMA, CL-imaging) and secondary ion mass spectrometry (SIMS) analysis has been applied to a representative number of Ca-phosphate grains. Analytical results for apatite and merrillite reveal: (i) zoning in F, Cl, Br and I concentrations, (ii) elevated Cl concentrations in the range of ∼11,900–35,300 µg/g and halogen ratios, i.e., Cl/Br and Cl/I, as well as stable chlorine isotope composition, reported as δ37Cl values rel. to Standard Mean Ocean Chloride (SMOC, defined as 0‰) with a value of +0.67 ± 0.14‰ (1σ), distinguishing KG 002 phosphates from that of other enriched and depleted shergottites. The halogen and heavier δ37Cl record indicate a slightly higher degree of ∼3.5% assimilation of Cl-rich and isotopically heavier crustal reservoir on Mars when compared to other enriched shergottites. (iii) Structural investigations together with the chemical and petrological context of the grains confirm the occurrence of hydroxyl-poor merrillite, indicate weak if any alteration effects induced by metamictization, only minor structural modifications due to shock metamorphism, and absence of replacement reactions. Therefore, igneous crystallization of Ca-phosphates from a fractionated, hydrous and ferrous mantle source, rich in volatiles including the halogens and Na and lithophile rare earth-elements, and absence of interaction with crustal fluids/brines of the sample is deduced. (iv) The Pb isotopic composition of six apatite and three merrillite grains is highly unradiogenic and the 238U-206Pb record yields a phosphate crystallization time at 395 ± 240 Ma (2σ), which is similar to those of other enriched shergottites.

Strong impact of ion filtration on the isotopic composition of chlorine in young clay-rich oceanic sediment pore fluids

A collection of 34 pore fluids from young clay-rich sedimentary piles of the Shikoku basin on the Philippine oceanic plate at the front of the Nankai subduction zone, southeast of Japan, were analyzed for their chloride content and chlorine stable isotope composition (35Cl and 37Cl, reported as δ37Cl). These pore fluids are samples from IODP Sites C0011 and C0012, which penetrate deeper than 500 meters below the seafloor. At both sites, the δ37Cl values display a regular decrease with depth from about 0‰ (seawater value) at the seafloor to −8.5 and −6.5‰ respectively in hydrogeologically isolated, low permeability, sediment levels. At some depths, local deviations of up to 6‰ in δ37Cl interrupt the overall decreasing trends. They may be interpreted as the injection of external fluids into the sedimentary piles as a result of sedimentary levels with higher permeability. At the bottom of the sedimentary piles, the injected fluids, probably originating from the altered oceanic crust, systematically have δ37Cl values closer to seawater. Literature data indicate the display of downward trends of decreasing δ37Cl values of the pore fluids in many other young clay-rich sedimentary piles (Nankai Trough and the Japan Trench accretionary prisms, Black Ridge and Juan de Fuca Ridge flank). Here, we use conservation equations to demonstrate that these δ37Cl depth profiles can be modeled using the effect of the compaction of growing clay–rich sedimentary piles on the water and chlorides (including 35Cl and 37Cl isotopes). This modeling shows that a small isotope fractionation of chlorine isotopes, α37Cl/35Clexpulsed fluid-residual fluid, in the range of 1.006 to 1.001, may explain the observed negative δ37Cl chlorides at the bottom of the sedimentary piles. The complementary positive δ37Cl chlorides must accumulate at the top but are very likely erased by dilution with seawater chlorides across the seafloor interface. This range of chlorine isotope fractionation values is in agreement with the theory of ion filtration of Phillips and Bentley (1987) in which the mobility of chlorides through semi-permeable clay membrane is determined by ion repulsion. Based on this study, we show that young pore fluids in clay-rich sediments of the oceanic crust are 37Cl-depleted chloride reservoirs. They might be the source of 37Cl-depleted chloride fluids advected at mud volcanoes on the seafloor but do not seem to contribute largely to subduction zone magmatic, tectonic, metamorphic products where no such 37Cl-depleted chlorine is documented. This in turn is a strong argument to propose that most of the clay-rich sediment’s pore waters are released back into the ocean rather than subducted.

Individual stages of bacterial dichloromethane degradation mapped by carbon and chlorine stable isotope analysis

The fractionation of carbon and chlorine stable isotopes of dichloromethane (CH2Cl2, DCM) upon dechlorination by cells of the aerobic methylotroph Methylobacterium extorquens DM4 and by purified DCM dehalogenases of the glutathione S-transferase family was analyzed. Isotope effects for individual steps of the multi-stage DCM degradation process, including transfer across the cell wall from the aqueous medium to the cell cytoplasm, dehalogenase binding, and catalytic reaction, were considered. The observed carbon and chlorine isotope fractionation accompanying DCM consumption by cell supensions and enzymes was mainly determined by the breaking of CCl bonds, and not by inflow of DCM into cells. Chlorine isotope effects of DCM dehalogenation were initially masked in high density cultures, presumably due to inverse isotope effects of non-specific DCM oxidation under conditions of oxygen excess. Glutathione cofactor supply remarkably affected the correlation of variations of DCM carbon and chlorine stable isotopes (Δδ13C/Δδ37Cl), increasing corresponding ratio from 7.2–8.6 to 9.6–10.5 under conditions of glutathione deficiency. This suggests that enzymatic reaction of DCM with glutathione thiolate may involve stepwise breaking and making of bonds with the carbon atom of DCM, unlike the uncatalyzed reaction, which is a one-stage process, as shown by quantum-chemical modeling.

A measurement method for isotope fractionation of 35Cl and 37Cl by a conventional through-diffusion experiment

Chlorine is geochemically stable and is widely used as a groundwater tracer. Stable isotopes of chlorine (35Cl and 37Cl) are fractionated by diffusion and are used to characterize diffusion-dominant systems, understand seawater intrusion and identify the origin of chlorine. However, a method for measuring the fractionation factor by diffusion (α = DCl−35 / DCl−37) has not been established for rock samples. We propose a method for measuring α using a conventional through-diffusion experiment, in which a sample is embanked by a high- to low-concentration tank. This study shows that theoretically, the diffusive flux in a through-diffusion experiment should have mass ratio fractionated by diffusion and that the difference between δ37Cl in the high- to low-concentration tank will approach to 103 ln α with time. The results of the through-diffusion experiment showed a significant difference between δ37Cl in the high- and low-concentration tanks. Measured α ranged from 1.0015–1.0022, which is in good agreement with previous studies. Moreover, the sampling period and correction method are suggested to determine α excluding errors due to an unsteady state. The proposed method is applicable to other isotopes to measure α and will contribute to a more quantitative evaluation of Cl and δ37Cl profiles.

Compound Specific Stable Chlorine Isotopic Analysis of Volatile Aliphatic Compounds Using Gas Chromatography Hyphenated with Multiple Collector Inductively Coupled Plasma Mass Spectrometry.

Stable chlorine isotope analysis is increasingly used to characterize sources, transformation pathways, and sinks of organic aliphatic compounds, many of them being priority pollutants in groundwater and the atmosphere. A wider use of chlorine isotopes in environmental studies is still inhibited by limitations of the different analytical techniques such as high sample needs, offline preparation, confinement to few compounds and mediocre precision, respectively. Here we present a method for the δ37Cl determination in volatile aliphatic compounds using gas chromatography coupled with multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS), which overcomes these limitations. The method was evaluated by using a suite of five previously offline characterized in-house standards and eight chlorinated methanes, ethanes, and ethenes. Other than in previous approaches using ICP methods for chlorine isotopes, isobaric interference of the 36ArH dimer with 37Cl was minimized by employing dry plasma conditions. Samples containing 2-3 nmol Cl injected on-column were sufficient to achieve a precision (σ) of 0.1 mUr (1 milliurey = 0.001 = 1‰) or better. Long-term reproducibility and accuracy was always better than 0.3 mUr if organics were analyzed in compound mixtures. Standardization is carried out by using a two-point calibration approach. Drift, even though very small in this study, is corrected by referencing versus an internal standard. The presented method offers a direct, universal, and compound-specific procedure to measure the δ37Cl of a wide array of organic compounds overcoming limitations of previous techniques with the benefits of high sensitivity and accuracy comparable to the best existing approaches.

Chlorine isotope fractionation between chloride (Cl−) and dichlorine (Cl2)

The use of chlorine stable isotopes (35Cl and 37Cl) can help to constrain natural processes that involve chlorine species with different oxidation states. Theoretical studies based on thermodynamic and quantum mechanical approaches predict large isotope fractionation during redox reactions but to date, these reactions have not been studied experimentally.Here, we explore the chlorine isotope fractionation during the oxidation of hydrated Cl− (redox state of -I) to Cl2 (redox state of 0) at 25°C and at 0°C. Our apparatus consists of a sealed glass reactor where liquid HCl is mixed with liquid H2O2, a strong oxidant. Following complex reaction pathways, this mixture ultimately leads to the oxidation of Cl− and to the formation of Cl2 gas. As long as it is degassing, the Cl2 gas is flushed out of solution using N2 as a vector-gas from the glass-reactor to a potassium hydroxide (KOH) solution (pH 14) where it disproportionates into soluble species: Cl− and ClO−. After each experiment, the chlorine isotopic composition was measured in the recovered KOH-trap solution, as well as in the residual HCl solution. Consistent with theoretical predictions, the produced Cl2 gas is always enriched in the heavier 37Cl as compared to the initial Cl-reservoir.The following isotope fractionation factors are obtained:At 0°C the isotopic fractionation 1000lnα(Cl2−Cl−) is 4.51 (+1.65/−0.49)‰At 25°C the isotopic fractionation 1000lnα(Cl2−Cl−) is 3.94 (+0.69/−0.18)‰.From the obtained data it is suggested that the production of Cl2 gas in our experiments is best described by a closed-system distillation. Our results are in agreement with published theoretical ab-initio calculations.

Chlorine Stable Isotope and Helium Isotope Studies Contributing to the Understanding of the Hydrogeochemical Characteristics of Old Groundwater

Chlorine stable isotope and helium isotope data were integrated in order to study the chemical and isotopic characteristics of old groundwater in the Pannonian Basin. Former studies showed groundwater is of meteoric origin, based on oxygen and deuterium isotopes, which have isotopic signatures of formation waters from the deeper part of the basin. Depletion of the heavier 37Cl isotope can be observed along the flow paths. Despite the fact that most of the sampled groundwater contains high helium levels, indicating a crustal helium origin, their 3He content is also high. The 3He/4He ratios (expressed in R/Rac) with values between 0.4 and 1.2 shows that helium of mantle origin must also be a component in the groundwater along the flow paths. The presence of mantle origin helium could be detected not just in the deeper part of the basin, but even in the Quaternary sediments.

Carbon, Hydrogen and Chlorine Stable Isotope Fingerprinting for Forensic Investigations of Hexachlorocyclohexanes.

Multielemental stable isotope analysis of persistent organic pollutants (POPs) has the potential to characterize sources, sinks, and degradation processes in the environment. To verify the applicability of this approach for source identification of hexachlorocyclohexane (HCHs), we provide a data set of carbon, hydrogen, and chlorine stable isotope ratios (δ13C, δ2H, δ37Cl) of its main stereoisomers (α-, β-, δ- and γ-HCHs) from a sample collection based on worldwide manufacturing. This sample collection comprises production stocks, agricultural and pharmaceutical products, chemical waste dumps, and analytical-grade material, covering the production time period from the late 1960s until now. Stable isotope ratios of HCHs cover the ranges from -233‰ to +1‰, from -35.9‰ to -22.7‰, and from -6.69‰ to +0.54‰ for δ2H, δ13C, and δ37Cl values, respectively. Four groups of samples with distinct multielemental stable isotope fingerprints were differentiated, most probably as a result of purification and isolation processes. No clear temporal trend in the isotope compositions of HCHs was found at the global scale. The multielemental stable isotope fingerprints facilitate the source identification of HCHs at the regional scale and can be used to assess transformation processes. The data set and methodology reported herein provide basic information for the assessment of environmental field sites contaminated with HCHs.

SIMS chlorine isotope analyses in melt inclusions from arc settings

The relative contribution of volatiles from altered oceanic crust, serpentinized mantle, and subducted sediments on the composition of primary arc magmas is poorly constrained. We measured the chlorine stable isotope composition in olivine-hosted melt inclusions in order to provide a first order constraint on the δ37Cl values of primary magmas, since melt inclusions are not or only little affected by near surface processes. Chlorine isotope analyses were obtained with a CAMECA IMS 1280-HR at the University of Lausanne. A series of six Cl-bearing glass standards with δ37Cl values of −1.1 to +1.7‰, SiO2 between 50 and 76wt% and Cl concentration between 0.15 and 3.25wt% were used for calibration. We determined that SiO2, Al2O3 and K2O in glasses are primarily responsible for the observed variations in instrumental mass fractionation.We obtained a δ37Cl reproducibility on the standard glasses typically better than 0.3‰ (2SD) and a total uncertainty, corresponding to the propagated error of SIMS and Pyro-IRMS uncertainty, of 0.4‰ (2SD). Melt inclusions from Lesser Antilles, Vanuatu and Aeolian arcs display ranges of δ37Cl values from −1.9 to +0.6‰, −1.7 to +0.4‰ and −3.4 to −1.4‰ respectively. Combined with Cl/K2O ratios, these data suggest that Cl addition mainly originates from serpentinites (lithospheric serpentinites or serpentinized mantle wedge) in the Lesser Antilles and Vanuatu arcs. In contrast, Cl added to the mantle wedge beneath the Aeolian Islands may record a higher proportion of fluids derived from the subducting sediments. The observed differences of δ37Cl values in melt inclusions from the Lesser Antilles and Vanuatu arcs and the Aeolian Islands may directly reflect the different subduction geometries present.

UV-FsLA-MC-ICP-MSを用いた高精度塩素同位体分析

UV-FsLA-MC-ICP-MSを用いた高精度塩素同位体分析

Stable chlorine isotopes in arid non-marine basins: Instances and possible fractionation mechanisms

Stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite. Halite and dissolved chloride in groundwater that has interacted with halite in arid non-marine basins has a δ37Cl range of 0 ± 3‰, far greater than the range for marine evaporites. Basins characterized by high positive (+1 to +3‰), near-0‰, and negative (−0.3 to −2.6‰) are documented. Halite in weathered crusts of sedimentary rocks has δ37Cl values as high as +5.6‰. Salt-excluding halophyte plants excrete salt with a δ37Cl range of −2.1 to −0.8‰. Differentiated rock chloride sources exist, e.g. in granitoid micas, but cannot provide sufficient chloride to account for the observed data. Single-pass application of known fractionating mechanisms, equilibrium salt-crystal interaction and disequilibrium diffusive transport, cannot account for the large ranges of δ37Cl. Cumulative fractionation as a result of multiple wetting-drying cycles in vadose playas that produce halite crusts can produce observed positive δ37Cl values in hundreds to thousands of cycles. Diffusive isotope fractionation as a result of multiple wetting-drying cycles operating at a spatial scale of 1–10 cm can produce high δ37Cl values in residual halite. Chloride in rainwater is subject to complex fractionation, but develops negative δ37Cl values in certain situations; such may explain halite deposits with bulk negative δ37Cl values. Future field studies will benefit from a better understanding of hydrology and rainwater chemistry, and systematic collection of data for both Cl and Br.

Experimental determination of stable chlorine and bromine isotope fractionation during precipitation of salt from a saturated solution

In order to better understand the chlorine and bromine stable isotope fractionation that occurs when chloride and bromide salts precipitate from their saturated solutions, we determined experimentally the equilibrium fractionation factors between precipitating pure salt minerals and their coexisting saturated brine at 22°C. Fractionation factors (expressed as 103lnα(37Cl/35Cl)salt-brine and 103lnα81Br/79Brsalt-brine) obtained for 11 chloride and 7 bromide salts of geological and industrial interest show a relatively largerange of variation (from −0.31 to +0.41), with the salt that precipitates having either a lower or a higher isotope ratio than the brine from which they precipitate. A negative fractionation factor indicates that the brine has a larger isotope ratio than the precipitate, a positive factor that the precipitate has a larger isotope ratio. The results of the chlorine and bromine isotope fractionation measurements (103lnα) for the various salts at a temperature of 22±2°C are: LiCl: +0.03±0.05, NaCl: +0.35±0.08, KCl: −0.12±0.05, NH4Cl: +0.09±0.03, RbCl: −0.31±0.05, CsCl: −0.23±0.02, MgCl2: −0.02±0.02, CaCl2: +0.04±0.02, SrCl2: +0.15±0.02, BaCl2: +0.41±0.03, FeCl3: −0.23±0.05, LiBr: −0.03±0.09, NaBr: −0.07±0.05, KBr: 0.00±0.01, NH4Br: +0.11±0.05, MgBr2: +0.06±0.06, CaBr2: −0.06±0.04 and SrBr2: +0.05±0.06. In these measurements the uncertainty is defined as the 1σ standard deviation of replicate determinations. We compare the results to previous theoretical calculations based on reduced partition coefficients as well as with thermodynamic properties of precipitated salts. Though most of our experimental fractionation data qualitatively agree with theoretical predictions (e.g., larger isotope fractionation for Cl than for Br species), it remains difficult to directly and finely compare those estimations which is likely due to the complexness of the processes that take place in saturated solutions. We use the data to predict the δ37Cl evolution of the Earth's oceans' through geological time under the condition that solely isotope fractionation during evaporation and salt precipitation took place, together with erosion of evaporite deposits. Under these conditions, we calculate a δ37Cl decrease of at most 0.25‰ for the Earth's oceans during the last 1billion years. Such a trend is not observed in old evaporite deposits, in part because δ37Cl variations in most evaporite deposits are larger than the expected trend, and we conclude that δ37Cl may also be influenced by other processes such as chloride exchange with the deep Earth. We only observed very small bromine isotope fractionation during precipitation of bromide salts from brine. This small fractionation cannot explain the large deviation of δ81Br of dissolved evaporite samples from precipitation of seawater with a modern bromine isotope composition, and suggests large bromine isotope variations in the oceans through time.

Stable chlorine isotope analysis of chlorinated acetic acids using gas chromatography/quadrupole mass spectrometry.

The environmental occurrence of chlorinated acetic acids (CAAs) has been extensively studied, but the sources and transport are still not yet fully understood. A promising approach for source apportionment and process studies is the isotopic characterization of target compounds. We present the first on-line stable chlorine isotope analysis of CAAs by use of gas chromatography/quadrupole mass spectrometry (GC/qMS). Following approved procedures for concentration analysis, CAAs extracted into MTBE were methylated to GC-amenable methyl esters (mCAAs). These mCAAs were then analyzed by GC/qMS for their stable chlorine isotope composition using a sample/standard-bracketing approach (CAA standards in the range δ(37) Cl -6.3 to -0.2 ‰, Standard Mean Ocean Chloride). Cross-calibration of the herein presented method with off-line reference methods (thermal ionization and continuous-flow GC isotope ratio mass spectrometry; TI-MS and CF-GC/IRMS, respectively) shows good agreement between the methods (regression slope for GC/qMS vs reference method data sets: 0.92 ± 0.29). Sample amounts as small as 10 pmol Cl can herewith be analyzed with a precision of 0.1 to 0.4 ‰. This method should be useful for environmental studies of CAAs at ambient concentrations in precipitations (<0.06 to 100 nmol L(-1) ), surface waters (<0.2 to 5 nmol L(-1) ) and soil (<0.6 to 2000 nmol kg(-1) dry soil) where conventional off-line methods cannot be applied.

Apatite as probe for the halogen composition of metamorphic fluids (Bamble Sector, SE Norway)

Halogen composition of replaced apatite formed during a regional metasomatic event (Bamble Sector, SE Norway) reveals information about the composition and evolution of the hydrothermal fluid. Infiltration and pervasive fluid flow of highly saline fluids into gabbroic bodies lead to scapolitization and amphibolitization, where magmatic Cl-rich apatite reacts with the hydrothermal fluid to form OH- and/or F-rich apatite. Apatite from highly altered samples adjacent to the shear zone has highest F (up to 15,000 µg/g) and lowest Br (4–25 µg/g) concentrations, whereas apatite from least altered samples has very low F (30–200 µg/g) and high Br (30–85 µg/g). In addition, individual replaced apatite grains show a zonation in F with high concentrations along rims and cracks and low F in core regions. Iodine concentrations remain rather constant as low values of 0.18–0.70 µg/g. We interpret all observed compositional features of replaced apatite to be the result of a continuous evolution of the fluid during fluid–rock interaction. Due to its high compatibility, F from the infiltrating fluid is incorporated early into recrystallized apatite (close to shear zone and rims of individual apatite grains). In contrast, Br as an incompatible halogen becomes enriched in the fluid and is highest in the most evolved fluid. Using experimental partition data between replaced apatite and fluid, we calculated F concentrations of the evolving fluid to decrease from 60 to <1 µg/g and Br to increase from ~1200 to ~5000 µg/g; I concentrations of the fluid are constant in the order of 370 µg/g. Although Cl is expected to show a similar behavior as Br, replaced apatite has constant Cl concentrations throughout the alteration sequence (~1 wt.%), which is likely the result of a rather constant Cl activity in the fluid. Chlorine stable isotope values of individual apatite grains are heterogeneous and range from −1.2 to +3.7 ‰. High δ 37Cl values are generally correlated with OH-rich zones of replaced apatite, whereas low δ 37Cl values are measured in F-rich zones of replaced apatite and in Cl-apatite of probably magmatic origin. Though apatite δ 37Cl values follow the general bulk trend, the individual δ 37Cl signature seems to reflect the highly localized composition of interfacial fluid at the reaction front. Our observations suggest that apatite can be used as a fluid probe for F, Br, and I to detect a compositional evolution of the fluid, which can be quantified by using experimentally derived partition coefficients. Partitioning of Cl and Cl stable isotopes between highly saline fluids and apatite is complex and likely controlled by more unknown factors than just the Cl concentration.

Pre-concentration of chloride in dilute water-samples for precise δ37Cl determination using a strong ion-exchange resin: Application to rainwaters

Low chloride concentration in water samples is a limitation for chlorine stable isotope measurements using a gas source isotope ratio mass spectrometer (IRMS). Here we describe an optimized method of chloride (Cl−) pre-concentration for dilute waters using a strong anion-exchange resin (Amberlite AG 1-X4). In order to validate our method and its reliability for δ37Cl measurements we made high precision δ37Cl measurements by dual inlet (DI) IRMS, also applicable to δ37Cl measurements by continuous flow IRMS.A 3ml solid phase extraction (SPE) cartridge is filled with 1ml of Amberlite resin in hydroxide form (100–200mesh) and is rinsed with 50ml of a 1M NaOH solution. The water sample containing aqueous chlorides is then loaded through the column at a flow rate of about 1.5ml/min. Because of the high partition coefficient of Cl− between resin and water (Kd>5300) the exchange of Cl− onto the resin is very efficient. We found that a volume as low as 5ml of a 1M KNO3 solution is enough for quantitative Cl− recovery from 1ml of resin. Using an internal laboratory standard solution as well as natural samples (seawaters and hydrothermal waters) with more complex chemistries and known δ37Cl values, the efficiency of this method was validated in terms of Cl− recovery yields, as well as accuracy and reproducibility of δ37Cl measurements.For dilute standard solutions with chloride concentrations ranging from 0.02 to 0.4mmol/l (Cl amounts ranging from 20 to 80μmol), the whole procedure recovery yields (ie. including the steps of Cl− pre-concentration and conversion into CH3Cl as well as consecutive CH3Cl purification) are 101±7% (1σ, n=21) and the measured δ37Cl agrees with the expected δ37Cl value to within ±0.04‰ (1σ), our current level of external precision reached by DI-IRMS measurements. Seawaters and hydrothermal waters with known δ37Cl values have also been diluted and treated using this method. All samples show quantitative Cl− recovery (yields~100%) and accurate δ37Cl values, with one maximum shift from the expected value of +0.09‰. We also found that for water samples containing relatively high concentrations of anions with a higher affinity for the resin than chlorides, such as sulfates or nitrates, the resin capacity may be exceeded, preventing a quantitative recovery of Cl− and yielding lower δ37Cl than expected. For such samples, we recommend increasing the volume of resin adequately. This method allows the investigation of as yet poorly documented δ37Cl in dilute natural waters (e.g., soil solutions, rainwaters and river waters) with highly precise DI-IRMS measurements. In a preliminary investigation, we applied our method to 6 rainwater samples collected at the summit and lowland of the Soufrière volcano. For Cl− concentrations between 0.06 and 0.1mmol/l we found small but significant δ37Cl variations, with values ranging from −0.16 to +0.38‰ (±0.04‰, 1σ). These results demonstrate that in active volcanic contexts, where rainwater Cl− may be partly sourced from volcanic degassing, δ37Cl carries information, which deserves further investigation given its potential implication for volcanic monitoring.

Chlorine isotope behavior during prograde metamorphism of sedimentary rocks

Chlorine stable isotope compositions of two sedimentary sequences and their metamorphic equivalents were measured in order to study fractionation effects during prograde metamorphism and devolatilization. Protoliths (n=25) were collected from a 50 m section of Triassic fluvial and playa-lake strata and Jurassic (Liassic) marine black shales in a well-characterized quarry. Low greenschist to middle amphibolite facies equivalents (n>80) were collected from the Glarus Alps, Urseren Zone, and Lucomagno region. Bulk δ37Cl values are constant within individual sedimentary layers, but vary from −2.0 to +2.4‰ in Triassic rocks and from −3.0 to 0‰ in the black shales. Dolomitic and gypsiferous samples have positive δ37Cl values, but marls and shales are isotopically negative. Bulk Cl contents show only small declines during the earliest stages of metamorphism. Metamorphic equivalents of the Triassic and Liassic protoliths record the same overall ranges in δ37Cl as their protoliths. Samples with highly correlated bulk compositions but different metamorphic grade show no statistically significant difference in δ37Cl. These data lead to the following conclusions: (1) Terrestrial and marine sedimentary rocks display large primary heterogeneities in chlorine isotope composition. As a result, an unambiguous “sedimentary signature” does not exist in the chlorine stable isotope system. (2) No isotopic fractionation is discernable during metamorphic devolatilization, even at low temperatures. Alpine-style metamorphism thus has little to no effect on bulk chlorine isotopic compositions, despite significant devolatilization. (3) Cl is largely retained in the rocks during devolatilization, contrary to the normally assumed hydrophilic behavior of chlorine. Continuous release of mixed-volatile C–O–H fluids likely affected Cl partitioning between fluid and minerals and allowed chlorine to remain in the rocks. (4) There is no evidence for fluid communication across (meta)sedimentary layers except within the Urseren shear zone. These data contribute to a relatively closed-system view of metamorphic devolatilization from anchizone through mid-amphibolite facies conditions.

Halogen Element and Stable Chlorine Isotope Fractionation Caused by Fluid–Rock Interaction (Bamble Sector, SE Norway)

Pervasive interaction between rock and fluid produces regional-scale alteration and/or metasomatism of rock units. The middle and upper crustal rocks from the Bamble sector (SE Norway) show a complex history of metamorphic and metasomatic events, accompanied by ore deposit formation, which provides an excellent context for investigation of regional-scale fluid‐rock interaction. Halogens are important agents of rock alteration and ore metal transportation in such hydrothermal regimes. Here we use F, Cl, Br and I concentrations and stable chlorine isotope ratios (d 37 Cl) to trace the metasomatic evolution of two gabbroic bodies and understand the interplay between localized and pervasive fluid flow in the upper crust. An alteration sequence was sampled from pristine gabbro to an amphibolitic shear zone, showing progressive amphibolitization and scapolitization caused by pervasive fluid ingress. Halogen concentrations and ratios suggest the evolution of a single fluid, causing enrichment of Cl, Br and I in samples nearest the shear zone. Owing to the differences in fluid‐mineral distribution coefficients, fluid‐mineral interaction resulted in either enrichment or depletion of halogens in the fluid as it reacted pervasively with the gabbro. Although scapolite formation leads to progressive desalination of the fluid near the shear zone, increasing Cl concentrations in amphibole near the gabbro reflect an increase in salinity towards the unaltered gabbro owing to desiccation of the fluid. The formation of alteration minerals mainly governs changes in the halogen chemistry of the fluid, showing that halogens do not behave fully conservatively during fluid‐rock interaction. Nevertheless, the evolving fluid has a feedback on the stability of alteration minerals such as scapolite and biotite, which form only close to the shear zone. The Br/Cl and I/Cl ratios of the most highly altered samples are 3 � 10 ‐3 and 25 � 10 ‐6 , respectively, and overlap with the range of ratios measured for marine pore fluids. Remobilization of evaporites would have produced distinctly lower halogen ratios and can be ruled out as a fluid source. The unaltered gabbro has d 37 Cl values near 0% and a similar value is inferred for the infiltrating fluid. Minimally altered samples, noted by the presence of <20 vol. % amphibole, have negative d 37 Cl values (average ¼ ‐0� 6 6 0� 1%). The d 37 Cl values increase (up to þ 1% nearest the shear zone) with increasing evidence of fluid‐rock interaction towards the shear zone. Mixing of different Cl reservoirs (i.e. gabbro and fluid) cannot explain the observed changes in the d 37 Cl values. Instead, isotopic fractionation owing to Rayleigh distillation with a fractionation factor of 1� 0010 can explain the observed negative d 37 Cl values of the alteration sequence by the evolution of a single fluid

A new high-precision method for determining stable chlorine isotopes in halite and igneous rock samples using UV-femtosecond laser ablation multiple Faraday collector inductively coupled plasma mass spectrometry

A new analytical protocol for measurement of stable chlorine isotopes was proposed for LA-MFC-ICPMS.